In the manufacture of ethylene polymers, ethylene gas is compressed into a supercritical fluid and then heated. The hot supercritical ethylene is then admitted into a tubular polymerization reactor, along with a supply of a chemical initiator and a modifier. The chemical initiator initiates polymerization of the free radical ethylene, while the modifier controls the molecular weight of the resulting polyethylene. Since only about 40% of the ethylene monomers react, the resulting polyethylene product that is discharged from the reactor is a mixture of ethylene polymers intermixed with unreacted ethylene. Consequently, it is necessary to separate the polymers from the ethylene. To this end, a high pressure separator vessel and a low pressure separator vessel are serially connected to the outlet of the polymerization reactor. The high pressure separator vessel initially receives the reactor product from the reactor at about 40,000 psi. The reactor contents are depressurized to about 4000 psi through a control valve into the high pressure separator vessel, which separates most of the polymer from the ethylene. The resulting polyethylene product still contains about 10% unreacted ethylene, and is admitted to the low pressure separator vessel. The lower pressure in this vessel results in the flashing away of the remainder of the unreacted ethylene from the product. The resulting polyethylene is then admitted into an extruder for final processing.
During processing, the outer walls of the low pressure separator vessel are continuously heated by means of a steam jacket in order to maintain the polyethylene product in a flowable liquid state. The applicants have observed that the non-Newtonian characteristics of the liquid polyethylene flowing through the low pressure separator vessel results in a very slow flow rate at the interface between the liquid polyethylene and inner surface of the vessel. The inner surface of the vessel is also where the interior temperature of the vessel is highest due to its closeness to the steam jacket that surrounds the exterior of the vessel. The combination of the high temperature of the vessel inner surface and the long residence time of the liquid polyethylene over it results in the production of degraded polymers on the inner surface due to thermally-induced, cross-linking reactions. If these degraded polymers are not periodically removed from the inner surfaces of the low pressure separator vessel, they can contaminate the final polyethylene product and degrade its appearance and film properties. The problem is worse in situations where a high clarity and purity polyethylene product is essential for the rendering of a particular final product, such as blown film products, medical applications and sensitive electrical applications.
To solve this problem, polyethylene manufacturers typically periodically clean the inner walls of the low pressure separator vessel by hydroblasting every several months. But because hydroblasting takes several days and must be done with the vessel in a horizontal position, most polyethylene manufactures replace the fouled low pressure separator vessel with a pre-cleaned, substitute separator vessel in order to reduce system downtime. Unfortunately, such a vessel replacement procedure still takes about a day to implement due to the time required to (1) mechanically disconnect all of the interfaces of the fouled vessel with the other components of the polymerization plant, (2) exchange the multi-ton fouled vessel with a multi-ton cleaned vessel and (3) to re-connect all of the interfaces between the clean vessel and the polymerization plant. Moreover, as the vessel weighs one or more tons, the step of exchanging the fouled vessel with a cleaned vessel must be done by way of a slow and delicate crane operation in order to avoid breakage or damage to the valves, pipes and other interface fittings that must be disconnected and reconnected.